The present invention may be characterized broadly as a moving floor type of conveyor. That is, due to the motion of the floor, objects placed on the floor at one end will be transported in a more or less straight line to the opposite end of the floor. The specific dimensions of the conveying surface portion of the apparatus depend upon the particular application. In one application, that surface may be relatively long and narrow so that it constitutes a track for transporting small objects through a processing zone or a work station. In another application such as a truck bed, that surface may be relatively wide to enable large objects such as crates to be offloaded from the truck.
The invention has particular application in connection with the transporting of relatively small, fragile articles such as semiconductor wafers through a high temperature processing zone. Accordingly, the invention will be described in that context. It should be viewed, however, in a much broader context as a conveyor for objects more or less of any size and shape.
There are basically three different types of moving floor transports. Perhaps the most common type is the vibratory feeder which comprises an article-supporting surface which is vibrated as part of a resonant spring mass system. While that type transport works satisfactorily, it has several disadvantages, particularly when used to move fragile objects such as semiconductor wafers in a laboratory environment. First, such apparatus tends to be relatively expensive because efforts must be made to isolate the vibrations produced by the apparatus. Despite those efforts, however, some vibratory motion is still coupled through the base of the apparatus into the building structure and to other apparatus and instruments in the vicinity, tending to upset the outputs and readings of those apparatus and instruments.
Also vibrating transports tend to be quite noisy so that they have a dibilitating effect on workers in the vicinity of the apparatus. Still further, vibrating transports being resonant systems can only transport objects at one speed and furthermore, they are not reversible so that the objects can only proceed in one direction on a given transport. Also such apparatus tends to be very large and massive because it requires a large support frame and a counterweight due to its mode of operation. Thus, for example, a four foot long vibratory transport weighing 100 lbs. requires a counterweight of at least 100 lbs. to provide the reaction force for the transport.
Vibrating type transports have particularly serious disadvantages when moving lightweight, fragile objects such as semiconductor wafers. This is because they advance the objects by bouncing those objects along the surface of the transport. Particularly in a high vacuum environment, the objects actually leave the surface and become airborne and then drop back onto the surface at a slightly advanced location thereon. Thus the objects are constantly striking the vibrating surface. Flat, lightweight objects such as semiconductor wafers tend to tilt as they become airborne so that the wafers often strike that surface slightly on edge causing edge chips and scratches which increase considerably the rejection rate for such wafers during processing.
On the other hand, if there is any liquid such as water and oil on the vibrating surface, flat lightweight articles such as semiconductor wafers tend to become adhered to the surface and may not bounce at all. Consequently, those wafers will not be advanced. In addition, the degree of advancement of objects on a vibrating transport depends upon the nature of the object. Thus a succession of semiconductor wafers of slightly different size are advanced by the transport at different rates so that one wafer may overrun another or lag excessively behind another. Finally such vibratory feeders advance whatever solid material is on the surface including dirt and dust particles and other debris which therefore enter the work station or processing zone along with the semiconductor wafers and may contaminate the process.
The second type of moving floor conveyor is the reciprocating slider transport described, for example, in U.S. Pat. No. 2,973,856. In that type conveyor, the surface is composed of an array of sliders or slats arranged side by side with the slats being reciprocated lengthwise in groups so that at any given time more slats are being advancing than are being retracted. Consequently, articles in frictional contact with the slats are moved in the direction of the advancing slats, which are in the majority. That type of transport also has several disadvantages which militates against its use as a conveyor for the small, lightweight, fragile articles of primary interest here. Most importantly, as some of the slats are advancing the objects, other slats are being retracted. These retracting slats inevitably scratch the undersides of the objects and if those objects are semiconductor wafers, ruining them. Also the reciprocating slats or sliders must be mounted in slides of some type and they are reciprocated by various kinds of lever and crank arrangements. To avoid undue parts wear, these slides and cranks must be kept lubricated. However, no know lubrication is able to withstand the high temperatures present in the processing zones used to fabricate semiconductors. Moreover, even if such lubrication could be developed, it would be volatile to some extent at those temperatures so that the emitted gases would contaminate the process being carried out in the working zone.
Also transports of the reciprocating slider type tend to be expensive because they require a large number of cranks, links, levers and other different parts which are relatively difficult and expensive to manufacture and maintain.
The third type of moving floor conveyor is the so-called walking beam conveyor described for example in U.S. Pat. No. 2,644,594. Its transporting surface is composed of an array of slats arranged side by side and divided into groups. In this case, however, one group of slats is raised up out of a reference plane so that it engages under objects on the conveyor, is advanced a short distance and then lowered below the reference plane. The objects supported by that group are thus deposited onto the remaining slats at a slightly advanced location along the conveyor surface. The second and third slat groups are moved in the same way in succession. Thus objects on the surface are lifted up, advanced and lowered repetitively in being conveyed from one end of the surface to the other.
A walking beam type conveyor has the same disadvantages discussed above in connection with the reciprocating slide conveyor. That is it is composed of a large number of different slides, links, levers, cranks and other such parts, all of which require lubrication, making such a conveyor totally unsuitable for use in semiconductor processing environments. So too, the walking beam conveyor is quite complex and expensive to make and maintain.